The manufacture of certain types of substrates is a time-consuming process which requires high levels of cleanliness. One example is substrates for integrated circuits. Many steps of manufacturing are conducted in various classes of so-called clean rooms, which have purified air flows to reduce the incidence of airborne particle contaminates. Nevertheless, wafers upon which semiconductor devices are fabricated can be rendered defective by contaminates introduced at various process steps.
In this regard, the manufacture of semiconductor devices typically involves process steps which are carried out on silicon wafers in process chambers, one example of which is an etching step. The level of particles in process chambers must be controlled, or else particles can be deposited on the wafers, thereby causing defects and significantly reducing the effective yield. In order to control the particle levels, process chambers are periodically subjected to a wet clean procedure, in which the chamber is disassembled or opened to the atmosphere, and then manually cleaned with a liquid such as water or isopropyl alcohol, in order to remove films from the chamber walls which contribute to the particle count. After such a wet clean, there will be an initial high level of particles for a brief period, as a result of the fact that the chamber has been opened to the atmosphere. However, in the process of resuming production, the level of particles will drop to a very low level as a result of the fact that the films removed from the chamber during the wet clean are no longer present to contribute particles.
After a wet clean, as production is carried out over time, the level of particles will begin to progressively increase, for example as films build back up on the chamber walls. In order to extend the time before the next wet clean must be carried out, it is possible to carry out one or more interim cleanings which do not require the use of liquids such as water or isopropyl alcohol.
One such interim approach, sometimes known as a cycle purge or pump purge, involves pumping the chamber pressure down, then raising the pressure by filling the chamber with a gas such as nitrogen or argon, and then pumping the chamber pressure back down. This cycle may be repeated several times. This approach is also used to reduce the particle levels following a wet clean and prior to resuming production. In any event, however, the time required to pump the chamber pressure down several times will usually necessitate an undesirable delay in production. This also means that the process chamber has a reduced availability to make products which can be sold.
Further, this technique essentially involves carrying out the purge cycle a predetermined number of times without taking any measurement of particle levels. If the entire procedure does not adequately reduce the particle levels, it is not immediately known that particle levels are unacceptable high. Alternatively, if particle levels are satisfactorily reduced substantially by the time the procedure is half complete, the procedure continues to completion anyway, which unnecessarily delays resumption of production. After completion of such a procedure, the particle levels may be measured by putting in the chamber some particle wafers that have been subjected to a pretest, then subjecting the particle wafers to a specified process, and then running a post-test analysis to see how many added particles are present. This is also a time-consuming technique, which delays the resumption of production. This same particle wafer test technique is also used to determine when to carry out the cycle purge procedure between wet cleans and, since it is a cumbersome technique, it is carried out relatively infrequently, which means that some production wafers will be processed in a chamber which is not suitably clean.
Another interim cleaning approach, commonly called a dry clean procedure, is to fill the chamber with inert gas such nitrogen or argon, and then strike a plasma to clean the chamber. Sometimes other gases such as oxygen are included, in order to assist in polymer removal. Depending on the particular process, such a dry clean procedure may be performed after processing of each wafer, only after processing each lot of wafers, or only on an "as-needed" basis. In any case, when the dry clean procedure is performed, the chamber is normally filled and the plasma struck only once, and the procedure does not involve any type of feedback regarding particle levels. After completion, particle levels in the chamber may be measured using particle wafers, with the associated disadvantages.
Yet another interim cleaning approach circulates a hot gas within the chamber, with forward and reverse gas flow. An in-situ particle monitor is provided to monitor particle levels. This process was developed primarily for use in eliminating moisture after a wet clean, rather than for reducing particle levels, although it does inherently have some effect in reducing particle levels. While a production mode of a process chamber typically involves successive production intervals for wafer processing which are spaced in time by nonproduction intervals, this hot gas circulation approach is not carried out during nonproduction intervals of the production mode.